1. Field of the Invention
The present invention relates to storage systems that store data to be used by a computer in a computer system, and more particularly to storage systems that use a cluster structure.
2. Related Background Art
As the amount of data treated in a computer system has drastically increased, the capacity of storage that stores data has been increased accordingly. As one of the approaches to reduce the management cost for large capacity storages, a storage system can be designed such that a large amount of data is collectively stored in a single large capacity storage of high performance, high reliability and high connectivity, instead of distributing a large amount of data and storing them in many small size storages. By reducing the number of storages that are subject to the management, the management cost for storages such as fault management and maintenance cost can be reduced.
However, when a single storage system is used to store a large amount of data, and is connected to many hosts, the number of control processors and storage devices that are mounted on the inside of the storage system increases. Furthermore, since a high speed linked operation is required among the numerous control processors and storage devices, the speed of internal buses and control memories needs to be increased and conflicts among many control processors need to be avoided. However, increasing the speed of internal buses and control memories and avoiding conflicts among control processors would present major challenges in terms of technology and costs in both hardware and software.
A cluster technology may be applied to a storage system in order to solve the problems described above and to realize a large capacity storage of a large scale and low cost.
A cluster storage system is typically composed of a plurality of relatively small scale storage nodes that are mutually connected through mutual connection systems such as switches to thereby realize a large capacity storage system. Many configurations are possible to compose such a large capacity storage system. However, they may be the same in that input/output requests received by a cluster storage system are distributed to storage nodes that are mounted with target devices of the input/output requests, and each of the input/output requests is processed by each of the corresponding storage nodes, respectively. Generally, a plurality of modules such as host interface, disk drive, control processors, memory, control memory, disk cache and the like are mounted on each of the storage nodes, like an ordinary storage system, and these modules are connected within the respective storage node via an internal network. At each of the storage nodes, these internal modules are used to process input/output requests with respect to the storage device.
In the cluster storage system described above, the disk cache and the control memory in each storage node are shared only by the control processors that are present in the same storage node. Therefore, in the cluster storage system described above, the performance requirements for internal buses and memory bandwidth can be alleviated. Furthermore, a plurality of storage nodes in the number that matches with a storage capacity required by a computer system may be connected to compose a cluster storage system. By so doing, scalable storage systems having a variety of different capacities from small capacity to large capacity can be realized. It is noted that, in a cluster storage system, control information and data stored in disk drives may be exchanged through a inter-connection system among storage nodes for data linkage such as data replication and internal data re-location among the disk drives.
When a new storage system is introduced in an existing computer system, it is effective, for reducing the device introduction cost, to utilize an existing storage that has already been introduced in the computer system by a customer. In order to migrate the entire data on the existing storage system to a new storage, the newly introduced storage system must be equipped with a capacity equivalent to that of the existing storage system, which increases the storage introduction cost.
As one of the known methods to connect a plurality of storage systems, a first storage system is connected to a second storage system, and a storage device (hereafter referred to as a “logical device”) that the first storage system provides to an upper device such as a host is provided to the host through the second storage system as a logical device of the second storage system. When the second storage system receives from the host an input/output request to access a logical device, the second storage system judges as to whether the storage device to be accessed corresponds to a logical device in the first storage system or a physical device within the second storage system, and sends the input/output request to an appropriate access destination according to the result of the judgment.
According to the conventional method described above, when the second storage system receives from the host an input/output request to access a logical device of the second storage system, which corresponds to a logical device of the first storage system (which may hereafter be referred to as an “external device”), the control processor of the second storage system judges that a device to be accessed is the external device, rewrites address information, and then transmits the input/output request to the first storage system. Accordingly, it is highly possible that the conventional method would affect the input/output processing performance for a device formed from the disk drive mounted within the second storage system (which may hereafter be referred to as an “internal device”).
For example, when an input/output processing for a logical device of the second storage system which corresponds to an external device of the first storage system is executed by the control processor of the second storage system, the operation capacity of the control processor of the second storage system is consumed.